Bacteria can respond to a variety of environmental stimuli by means of systems generally composed of two proteins. The first protein (sensor or transmitter) is usually a transmembrane protein with cytoplasmic and extracytoplasmic domains. The extracytoplasmic domain (sensor) senses the environment and transfers the signal through the transmembrane domain to the cytoplasmic domain (transmitter), which has kinase activity. The second protein is located in the cytoplasm and contains an amino-terminal domain (receiver), which can be phosphorylated by the transmitter, and a carboxy-terminal region (regulator), which regulates gene expression by binding to DNA. The transmitter and receiver modules (the kinase and its target) are conserved in all signal-transducing systems and are the 'core structure' of this two-component system. The sensors and the regulators vary according to the stimuli they respond to and the DNA structure they interact with. On the basis of their sequence homology, the proteins belonging to such two-component systems can be classified into different families, which are summarized in this review.